As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased as an energy source for the mobile devices. Among such secondary batteries is a lithium secondary battery having high energy density and high discharge voltage, into which much research has been carried out and which is now commercially and widely used.
Depending upon the shape of a battery case, a secondary battery may be classified as a cylindrical battery having an electrode assembly mounted in a cylindrical metal container, a prismatic battery having an electrode assembly mounted in a prismatic metal container, or a pouch-shaped battery having an electrode assembly mounted in a pouch-shaped case formed of an aluminum laminate sheet. The cylindrical battery has advantages in that the cylindrical battery has relatively large capacity and is structurally stable.
Also, the electrode assembly mounted in the battery case serves as a power generating element, having a cathode/separator/anode stack structure, which can be charged and discharged. The electrode assembly may be classified as a jelly roll type electrode assembly configured to have a structure in which a long sheet type cathode and a long sheet type anode, to which active materials are applied, are wound while a separator is disposed between the cathode and the anode or a stacked type electrode assembly configured to have a structure in which pluralities of cathodes and anodes having a predetermined size are sequentially stacked while separators are disposed respectively between the cathodes and the anodes. The jelly roll type electrode assembly has advantages in that the jelly roll type electrode assembly is easy to manufacture and has high energy density per unit mass.
FIG. 1 is a vertical sectional perspective view typically showing a general cylindrical battery.
Referring to FIG. 1, a cylindrical battery 100 is manufactured by inserting a jelly roll type (wound type) electrode assembly 120 into a cylindrical case 130, injecting an electrolyte into the cylindrical case 130, and coupling a top cap 140 having an electrode terminal (not shown), for example a cathode terminal, to the upper end, which is open, of the cylindrical case 130.
The jelly roll type electrode assembly 120 is configured to have a structure in which a cathode 121 and an anode 122 are wound in a circle in a state in which a separator 123 is disposed between the cathode 121 and the anode 122. A cylindrical center pin 150 is disposed at the center of the roll, i.e. the center of the electrode assembly 120. The center pin 150 is generally made of a metal material to exhibit predetermined strength. The center pin 150 is configured to have a hollow cylindrical structure formed by rolling a metal sheet. The center pin 150 serves to fix and support the electrode assembly 120. In addition, the center pin 150 serves as a passage to discharge gas generated by internal reaction of the secondary battery when charging and discharging the secondary battery or when operating the secondary battery.
Meanwhile, a lithium secondary battery has a disadvantage in that the lithium secondary battery has low safety. For example, when the battery is overcharged to approximately 4.5 V or more, a cathode active material is decomposed, dendritic growth of lithium metal occurs at an anode, and an electrolyte is decomposed. At this time, heat is generated from the battery with the result that the above-mentioned decompositions and several sub decompositions rapidly progress, and, eventually, the battery may catch fire and explode.
In order to solve the above-mentioned problems, therefore, a general cylindrical secondary battery includes a current interruptive device (CID) mounted in a space defined between a jelly roll type electrode assembly and a top cap to interrupt current and release internal pressure when the secondary battery malfunctions.
A series of operations performed by such a CID is shown in FIGS. 2 to 4.
Referring to these drawings, a top cap 10 protrudes to form a cathode terminal. The top cap 10 has exhaust ports. Below the top cap 10 are sequentially disposed a positive temperature coefficient (PTC) element 20 to interrupt current through significant increase of battery resistance when the interior temperature of the battery increases, a safety vent 30 configured to have a downwardly depressed shape in a normal state and to protrude and rupture for exhausting gas when the interior pressure of the battery increases, and a current interruptive device 50 coupled to the safety vent 30 at one side of the upper end thereof and connected to a cathode of an electrode assembly 40 at one side of the lower end thereof. Also, the outer circumference of the current interruptive device 50 is surrounded by a current interruptive device gasket 52 to fix the current interruptive device 50.
Under normal operating conditions, therefore, the cathode of the electrode assembly 40 is connected to the top cap 10 via an electrode lead 42, the current interruptive device 50, the safety vent 30, and the PTC element 20 to achieve electric conduction.
However, when gas is generated from the electrode assembly 40 for various reasons, such as overcharging, the internal pressure of the battery increases, and the shape of the safety vent 30 is inversed as shown in FIG. 3. That is, the safety vent 30 protrudes upward. At this time, the safety vent 30 is separated from the current interruptive device 50 to interrupt current. As a result, the overcharging is prevented from further progressing, thereby achieving safety. When the internal pressure of the battery continues to increase, however, the safety vent 30 ruptures, as shown in FIG. 4, with the result that the pressurized gas is discharged through the exhaust ports of the top cap 10 via the ruptured safety vent 30, thereby preventing explosion of the battery.
Meanwhile, the cylindrical secondary battery must exhibit a high temperature storage property in addition to the above-described overcharging property.
The high temperature storage property entails a unique property of a battery in which voltage of the battery does not fall when a battery is maintained at a predetermined temperature (for example, 75° C.) for a long period of time. Specifically, the high temperature storage property entails operation of the battery at normal voltage and current even after testing.
The overcharging property entails normal operation of a CID to effectively interrupt voltage and current, thereby preventing explosion of a battery before the battery is exploded due to increase in internal pressure and temperature of the battery when the battery is charged to a higher level than normal voltage.
A method of adjusting an amount of an electrolyte injected within a short circuit pressure range of 8 to 14 kgf/cm2 is mainly used to satisfy both the high temperature storage property and the overcharging property.
If a small amount of the electrolyte is injected into the cylindrical battery container, the high temperature storage property becomes advantageous but the overcharging property becomes disadvantageous. That is, if the amount of the electrolyte is small, the internal space of the battery cell is increased. If gas is generated in the battery cell in a state in which the battery cell is overcharged, therefore, the internal temperature of the battery cell is increased before sufficient gas to rupture the CID is generated with the result that the battery cell is exploded.
On the other hand, if a large amount of the electrolyte is injected into the cylindrical battery container, the overcharging property becomes advantageous but the high temperature storage property becomes disadvantageous. That is, if the amount of the electrolyte is large, the internal space of the battery cell is decreased. If gas is generated in the battery cell in a state in which the battery cell is at a high temperature, therefore, the CID is ruptured with the result that the battery cell cannot be used.
Consequently, there is a high necessity for a cap assembly of a novel structure that is capable of simultaneously satisfying a high temperature storage property and an overcharging property without adjusting an amount of an electrolyte injected and a cylindrical battery including such as cap assembly.